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Debugging CoreCLR

These instructions will lead you through debugging CoreCLR on Windows and Linux. They will be expanded to support OS X when we have good instructions for that.

Debugging CoreCLR on Windows

  1. Perform a build of the repo.
  2. Open <repo_root>\bin\obj\Windows_NT.<platform>.<configuration>\CoreCLR.sln in VS. <platform> and <configurtion> are based on type of build you did. By default they are 'x64' and 'Debug'.
  3. Right click the INSTALL project and choose ‘Set as StartUp Project’
  4. Bring up the properties page for the INSTALL project
  5. Select Configuration Properties->Debugging from the left side tree control
  6. Set Command=$(SolutionDir)..\..\product\Windows_NT.$(Platform).$(Configuration)\corerun.exe
    1. This points to the folder where the built runtime binaries are present.
  7. Set Command Arguments=<managed app you wish to run> (e.g. HelloWorld.exe)
  8. Set Working Directory=$(SolutionDir)..\..\product\Windows_NT.$(Platform).$(Configuration)
    1. This points to the folder containing CoreCLR binaries.
  9. Press F11 to start debugging at wmain in corerun (or set a breakpoint in source and press F5 to run to it)
    1. As an example, set a breakpoint for the EEStartup function in ceemain.cpp to break into CoreCLR startup.

Steps 1-8 only need to be done once, and then (9) can be repeated whenever you want to start debugging. The above can be done with Visual Studio 2013.

Debugging CoreCLR on OS X

To use lldb on OS X, you first need to build it and the SOS plugin on the machine you intend to use it. See the instructions in building lldb. The rest of instructions on how to use lldb for Linux on are the same.

Debugging CoreCLR on Linux

Only lldb is supported by the SOS plugin. gdb can be used to debug the coreclr code but with no SOS support. Visual Studio 2015 RTM remote debugging isn't currently supported.

  1. Perform a build of the coreclr repo.
  2. Install the corefx managed assemblies to the binaries directory.
  3. cd to build's binaries: cd ~/coreclr/bin/Product/Linux.x64.Debug
  4. Start lldb (the version the plugin was built with, currently 3.6): lldb-3.6 corerun HelloWorld.exe linux
  5. Now at the lldb command prompt, load SOS plugin: plugin load libsosplugin.so
  6. Launch program: process launch -s
  7. To stop annoying breaks on SIGUSR1/SIGUSR2 signals used by the runtime run: process handle -s false SIGUSR1 SIGUSR2
  8. Get to a point where coreclr is initialized by setting a breakpoint (i.e. breakpoint set -n LoadLibraryExW and then process continue) or stepping into the runtime.
  9. Run a SOS command like sos ClrStack or sos VerifyHeap. The command name is case sensitive.

You can combine steps 4-8 and pass everything on the lldb command line:

lldb-3.6 -o "plugin load libsosplugin.so" -o "process launch -s" -o "process handle -s false SIGUSR1 SIGUSR2" -o "breakpoint set -n LoadLibraryExW" corerun HelloWorld.exe linux

SOS commands

This is the full list of commands currently supported by SOS. LLDB is case-sensitive unlike windbg.

Type "soshelp <functionname>" for detailed info on that function.

Object Inspection                  Examining code and stacks
-----------------------------      -----------------------------
DumpObj (dumpobj)                  Threads (clrthreads)
DumpArray                          ThreadState
DumpStackObjects (dso)             IP2MD (ip2md)
DumpHeap (dumpheap)                u (clru)
DumpVC                             DumpStack (dumpstack)
GCRoot (gcroot)                    EEStack (eestack)
PrintException (pe)                ClrStack (clrstack)
                                   GCInfo
                                   EHInfo
                                   bpmd (bpmd)

Examining CLR data structures      Diagnostic Utilities
-----------------------------      -----------------------------
DumpDomain                         VerifyHeap
EEHeap (eeheap)                    FindAppDomain
Name2EE (name2ee)                  DumpLog (dumplog)
DumpMT (dumpmt)
DumpClass (dumpclass)
DumpMD (dumpmd)
Token2EE
DumpModule (dumpmodule)
DumpAssembly
DumpRuntimeTypes
DumpIL (dumpil)
DumpSig
DumpSigElem

Examining the GC history           Other
-----------------------------      -----------------------------
HistInit (histinit)                FAQ
HistRoot (histroot)                Help (soshelp)
HistObj  (histobj)
HistObjFind (histobjfind)
HistClear (histclear)

Aliases

By default you can reach all the SOS commands by using: sos [command_name] However the common commands have been aliased so that you don't need the SOS prefix:

bpmd            -> sos bpmd
clrstack        -> sos ClrStack
clrthreads      -> sos Threads
clru            -> sos U
dso             -> sos DumpStackObjects
dumpclass       -> sos DumpClass
dumpheap        -> sos DumpHeap
dumpil          -> sos DumpIL
dumplog         -> sos DumpLog
dumpmd          -> sos DumpMD
dumpmodule      -> sos DumpModule
dumpmt          -> sos DumpMT
dumpobj         -> sos DumpObj
dumpstack       -> sos DumpStack     
eeheap          -> sos EEHeap
eestack         -> sos EEStack
gcroot          -> sos GCRoot
histinit        -> sos HistInit
histroot        -> sos HistRoot
histobj         -> sos HistObj
histobjfind     -> sos HistObjFind
histclear       -> sos HistClear
ip2md           -> sos IP2MD
name2ee         -> sos Name2EE
pe              -> sos PrintException
soshelp         -> sos Help

Problems and limitations of lldb and SOS

Many of the SOS commands like clrstack or dso don't work on core dumps because lldb doesn't return the actual OS thread id for a native thread. The "setsostid" command can be used to work around this lldb bug. Use the "clrthreads" to find the os tid and the lldb command "thread list" to find the thread index (#1 for example) for the current thread (* in first column). The first setsostid argument is the os tid and the second is the thread index: "setsosid ecd5 1".

The "gcroot" command either crashes lldb 3.6 or returns invalid results. Works fine with lldb 3.7 and 3.8.

Loading Linux core dumps with lldb 3.7 doesn't work. lldb 3.7 loads OS X and FreeBSD core dumps just fine. lldb 3.8 loads all the platform's core dumps without problem.

For more information on SOS commands see: https://msdn.microsoft.com/en-us/library/bb190764(v=vs.110).aspx

Debugging Mscorlib and/or managed application

To step into and debug managed code of Mscorlib.dll (or the managed application being executed by the runtime you built), using Visual Studio, is something that will be supported with Visual Studio 2015. We are actively working to enable this support.

Until then, you can use WinDbg and SOS (an extension to WinDbg to support managed debugging) to step in and debug the generated managed code. This is what we do on the .NET Runtime team as well :)